This invention relates to signal generators, and is particularly applicable to signal generators which are intended to perform measurements and tests. Such instruments are required to produce an output signal having precisely specified characteristics, such as carrier frequency, modulation depth or power level. It is particularly important to achieve and maintain specified levels of precision when testing RF components or systems.
Increasingly the devices under test require the use of more than one signal generator in order to carry out a test which simulates typical operating conditions. At present, this kind of requirement is met by the use of a number of independent signal generators, the output signals of which are adjusted so as to have the desired relationship with each other. Not only can this be time consuming, it can also be difficult to achieve the required degree of accuracy, and to maintain that accuracy over a period of time.
For example, two or three tone intermodulation measurements on amplifiers need to be carried out where an amplifier, such as those used in cable television systems, is required to amplify two or more signals. All amplifiers introduce some distortion to the signals and when two or more signals are being used the non linearities cause the generation of spurious signals at other frequencies. These signals can be highly undesirable since they can cause interference to users at other frequencies. If, for instance, an amplifier is passing signals at 600 MHz and 610 MHz, then the spurious signals are generated primarily at frequencies of 590 MHz and 620 MHz and users of these frequencies would experience interference.
FIG. 1 shows how an amplifier can be tested using RF signal generators. Two (or more) signal generator outputs are combined together to produce RF signals with equal levels. The signals are then passed through the amplifier and the output spectrum is monitored by a suitable signal analyzer to measure the spurious signals relative to the wanted signals.
Another example of where two or more signal generators are required, is in the testing of an RF mixer, which is a device which can be used to change the frequency of a signal. If an input signal of, for instance 600 MHz is applied to the RF port of the mixer and a local oscillator of, for instance 610 MHz is applied to the LO port, it will produce signals at the sum (1210 MHz) and the difference (10 MHz) frequencies. The output can then be filtered to remove unwanted signals. In this example, if only the difference frequency is required, the signal could be put through a low pass filter to remove any oscillator breakthrough signals and the sum component.
Frequency converters on communication system are often required to work with input signals which have more than one principal component. In this example, the mixer could be required to convert two signals, 610 MHz and 611 MHz, to a frequency of 10 MHz and 11 MHz respectively. As with testing amplifiers, however, the mixer is not perfectly linear and this non linearity will result in the generation of intermodulation signals at 9 MHz and 12 MHz on the IF output.
FIG. 2 shows how signal generators can be used to test a mixer to check its intermodulation performance. One signal generator is used to provide the local oscillator signal to drive the mixer. The second and third signal generator outputs are combined in a suitable summing network and the combined output signal is applied to the RF port of the mixer. The output of the mixer is then measured using a suitable signal analyzer to measure the intermodulation products generated in the mixer.
Receivers in communications systems have to be able to receive weak transmitted signals when other stronger transmitters being received. An example of this is that receivers operating in the avionics bands above 108 MHz must be able to work when much more powerful signals from FM broadcast transmitters that are close by. The ability of a receiver to reject these signals and still receive the wanted signal is a measure of the selectivity (or blocking ratio) of the receiver. The measurement is usually quoted as the relative signal level of the unwanted signal compared to the wanted signal when the receiver is meeting a specified performance criteria (e.g. signal to noise ratio or bit error rate).
The selectivity can be measured as shown in FIG. 3. Two signal generator outputs are combined in a suitable summing network and the output is applied to the input of the receiver. The signal level of the first signal generator is set at a low RF level with a modulated carrier so that the receiver is operating with, for instance, an 18 dB signal to noise ratio. The interfering signal is then set to a frequency at which it is necessary to check the selectivity and the level is increased until the receiver performance degrades to, for instance, 12 dB. The difference in levels between the two signals at the receiver input is the selectivity of the receiver.
There are many other applications for signal generators which require the use of more than one signal generator to undertake a test. However, the use of separate signal generators, external cables and combining networks combined with the voltage standing wave ratio or VSWR or of the components and the signal generators can generate significant errors in the measurement which the user has to allow for. In addition, different applications require that the signal generators are physically connected together in different ways in order to undertake the test. The different test applications also require that the signal generator parameters need to interact with each in different ways according to the test being carried. In the case of intermodulation, for instance, the two signals need to always have the same RF level and frequency difference, while selectivity measurements require the frequency separation and the level of the interferer relative to the in-channel signal to be variable. This can require that parameters on both signal generators need adjusting and the interaction is dependent on the test application.